Real & Simulated QPSK Up-Converted Signals by a Sampling Method Using a Cascaded MZMs Link

Round 1

Reviewer 1 Report

The manuscript entitled “Real & Simulated QPSK Up-Converted Signals by a Sampling Method Using a Cascaded MZMs Link” demonstrated a sampling up-conversion technique based on cascaded MZMs. Error vector magnitude (EVM) simulations and experiments have been conducted and compared to verify the performance of the system. The paper can be accepted for publication in Photonics if the authors can properly address the following concerns.

1. The cascaded MZM for frequency up-conversion is widely investigated. I believe the manuscript is lack of novel concept and technique unless the authors can provide distinct advantages comparing with other similar schemes.
2. As the principle of the MZM is familiar to most of the researchers in photonics, I believe the section 2 and section 3 can be refined.
3. What is the function of the Att1 and Att2 in Fig. 7? What kind of optical filter is used? Why the MZM1 in Fig. 7 is driven by a signal with the frequency of 0.5 GHz, but the f_IF is 1 GHz in Fig. 8?
4. The authors mentioned that up-converted signal degrades in the experiments because of the thermal noise and loss of the single mode fiber, I believe it would be more beneficial if some related references can be provided.
5. The manuscript can be improved as some typos and mistakes can be found.

Author Response

Dear Professor,

I would like to thank you for your various and valuable comments and suggestions that allowed us to greatly improve the quality of our manuscript. The modified version of our manuscript incorporates many changes to reflect the requested suggestions. 

Best regards,

Hassan TERMOS

Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, a cascaded Mach-Zehnder modulators (MZMs) link is used to transmit of a quadrature phase shift keying (QPSK) modulation signal by electro-optical frequency up-conversion. Related theoretical analysis, simulation and experimental results are presented. It is clearly organized and has its own special features. Here are some suggestions for this paper:

1. Figure 1 from the software VPI is not much clear. It is recommended the authors to draw a clear schematic diagram, and the font marked in the picture should be consistent with other figures. The connection lines output from MZM should be drawn in standard in Figure 3. The horizontal axis of the coordinate axis should be at 0 dBm, and the unit fonts for marking the optical signal power should be the same in Figure 4.
2. The abbreviations first-time mentioned in the text should be explained, such as AWG, EVM, LNA, etc. In the following description, the co-occurrence of abbreviations and phrases makes the paper looks like messy.
3. The order of the abstract content should correspond to the order of the text. The analysis of the MZM modulator described at the end of the Abstract does not correspond to the text. The third part in the paper analyzes the design and operating regime peculiarities of the MZM, so it is suitable to put in the corresponding position in the abstract.
4. The keywords Electro-Optical mixer and Mach-Zehnder Modulation have the same meaning, and the content of EOM rarely appears in the full paper. Please consider another keywords in the paper.
5. When describing "various optical mixers", more than one device should be list behind in the 35-36^th rows.
6. Why does the sampling signal module in the article use such a structure to generate a 7.8-GHz signal? Such level of frequency could be directly generated using a general signal source.
7. Relevant parameter information in experiments and simulations is missing, such as the bias voltages V_b1 and V_b2 of MZM1 and MZM2.
8. When evaluating the quality of the QPSK signal, EVM is an important criterion. The EVM limit of the QPSK data with 17.5% seems a little high in this paper. Is it enough for most applications? What’s more, it is suggested to add a constellation diagram, which can more intuitively reflect the experimental results.
9. Both experiments and simulations are based on principle analysis, and the formula deduction process should be attached. Such as the formula before the MZM1, after the MZM1, after the MZM2, and after the PD. This will make the paper more authoritative and persuasive.
10. It is verified that the best bias voltage is 3.6 v. As far as I know, this parameter can be set directly in the VPI, and the bias voltage of the device will also be given in the experiment. If the significance of this verification test is to detect the MZM, it should be placed before all experiments. It seems meaningless to verify at the end.

Author Response

Dear Professor,

I would like to thank you for your various and valuable comments and suggestions that allowed us to greatly improve the quality of our manuscript. The modified version of our manuscript incorporates many changes to reflect the requested suggestions. 

Best regards,

Hassan TERMOS

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have addressed all my concerns and the paper can be accepted in present form.

Reviewer 2 Report

The authors have carefully made revision and the manuscript can be considered for publication.